Ever take a whiff of a smoker's hair and feel faint from the pungent scent of cigarette smoke? Or perhaps you have stepped into an elevator and wondered why it smells like someone has lit up when there is not a smoker in sight. Welcome to the world of third-hand smoke.

"Third-hand smoke is tobacco smoke contamination that remains after the cigarette has been extinguished," says Jonathan Winickoff, a pediatrician at the Dana–Farber/Harvard Cancer Center in Boston and author of a study on the new phenomenon published in the journal Pediatrics. According to the study, a large number of people, particularly smokers, have no idea that third-hand smoke—the cocktail of toxins that linger in carpets, sofas, clothes and other materials hours or even days after a cigarette is put out—is a health hazard for infants and children. Of the 1,500 smokers and nonsmokers Winickoff surveyed, the vast majority agreed that second-hand smoke is dangerous. But when asked whether they agreed with the statement, "Breathing air in a room today where people smoked yesterday can harm the health of infants and children," only 65 percent of nonsmokers and 43 percent of smokers answered "yes."

"Third-hand smoke," a term coined by Winickoff's research team, is a relatively new concept but one that has worried researchers and nonsmokers for several years. "The third-hand smoke idea—concern over that—has been around for a long time. It's only recently been given a name and studied," says Stanton Glantz, director of the Center for Tobacco Control Research and Education at the University of California, San Francisco. "The level of toxicity in cigarette smoke is just astronomical when compared to other environmental toxins [such as particles found in automobile exhaust]," he adds, but notes that he is not aware of any studies directly linking third-hand smoke to disease [as opposed to second-hand smoke, which has been associated with disease].

ScientificAmerican.com asked Winickoff to explain exactly what third-hand smoke is and why it poses a public health risk.

How exactly do you distinguish between second- and third- hand smoke?
Third-hand smoke refers to the tobacco toxins that build up over time—one cigarette will coat the surface of a certain room [a second cigarette will add another coat, and so on]. The third-hand smoke is the stuff that remains [after visible or "second-hand smoke" has dissipated from the air]…. You can't really quantify it, because it depends on the space…. In a tiny space like a car the deposition is really heavy…. Smokers [may] smoke in another room or turn on a fan. They don't see the smoke going into a child's nose; they think that if they cannot see it, it's not affecting [their children].

Why is third-hand smoke dangerous?
The 2006 surgeon general's report says there is no risk-free level of tobacco exposure…. There are 250 poisonous toxins found in cigarette smoke. One such substance is lead. Very good studies show that tiny levels of exposure are associated with diminished IQ.

What do you consider the most dangerous compound in cigarette smoke?
I would say cyanide, which is used in chemical weapons. It actually interferes with the release of oxygen to tissues. It competitively binds to hemoglobin [meaning it competes with oxygen for binding sites on the blood's oxygen-carrying molecule, hemoglobin]. Basically people with cyanide poison turn blue…. [And] arsenic, that is a poison used to kill mammals. We [used to] use it to kill rats. And there it is in cigarette smoke.

Why are the risks associated with exposure to third-hand smoke different for children and adults?
The developing brain is uniquely susceptible to extremely low levels of toxins. Remember how we talked about the layers of toxin deposits on surfaces? Who gets exposure to those surfaces? Babies and children are closer to [surfaces such as floors]. They tend to touch or even mouth [put their mouths to] the contaminated surfaces. Imagine a teething infant.

What types of places or materials harbor the greatest amount of third-hand smoke?
Anywhere you see an enclosed space you should watch out for [it].

By introducing the phrase "third-hand smoke" in your research, what do you hope to accomplish?
This study points to the need for every smoker to try to quit. That's the only way to completely protect their children…. Really, I think that what this says is that we need to have sympathy for smokers and help them quit smoking…. [And also] that the introduction of this concept will lead to more smoke-free spaces in…public.

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